The present invention relates to an optical beam diameter reducer, more specifically to an optical beam diameter reducer for reducing a beam diameter of an optical beam.
Fiber type optical devices such as a fiber type optical amplifier, a fiber laser and a fiber type nonlinear element need to raise optical power density inside a core for obtaining high-output, improved pumping efficiency or increased nonlinear effect. In order to increase the optical power density, a core diameter should be reduced or optical power of input light should be increased. However, as the core diameter becomes smaller, to increase the input light power becomes more difficult. Accordingly, it has been expected to develop a method that makes it possible to apply high electric power into an optical fiber with a small core diameter.
From the viewpoint of high output, a multimode laser diode (LD) is most desirable among conventional optical sources. When high output light of the multimode LD efficiently enters into an optical fiber having a small core diameter such as a single-mode optical fiber, extremely high optical power density is obtained. Therefore, a high output fiber type optical amplifier, a high output fiber laser and a highly effective fiber type nonlinear element can be realized.
However, since the output light from the multimode LD is large in diameter ranging from several tens xcexcm to 200 xcexcm, it is very difficult to enter it efficiently into a fiber core with a small core. In order to solve this problem, a taper type optical fiber has been proposed which core gradually tapers off from an input end to an output end. That is, in the taper type optical fiber, light entered from the input end with a large core diameter is converted into a waveguide mode with a small diameter while propagating in the fiber core which diameter gradually tapers off. When the light finally outputs from the output end with a small core diameter, its beam diameter becomes suitable for efficiently connecting with the following optical fiber.
As to a conventional taper type optical fiber, although it is possible to convert from a higher mode to a lower mode, the conversion from the higher mode into a single mode is difficult. Namely, when it is connected with a single mode optical fiber, high coupling efficiency could not be obtained.
Furthermore, since the core diameter of the taper type optical fiber varies in the longitudinal direction, it cannot be mass-produced by fiberdrawing from a single preform like ordinary optical fibers and thus has to be manufactured individually one by one. As a result, the taper type optical fibers are high-priced.
An object of the present invention is to solve the aforementioned problems and provide an optical beam diameter reducer for efficiently reducing a large diameter of an optical beam.
Another object of the present invention is to provide an optical beam diameter reducer which can be produced at a low price.
Further object of the present invention is to provide an optical beam diameter reducer which can be mass-produced by fiberdrawing from a single preform.
An optical beam diameter reducer according to the invention comprises a core with the highest refractive index; a refractive index inclined layer formed outside the core which refractive index is lower than that of the core and gradually reduces as approaching the outside in the radial direction; and a low refractive index layer formed outside the refractive index inclined layer which refractive index is lower than the lowest refractive index of the refractive index inclined layer.
Owing to the refractive index inclined layer, an optical beam of a large diameter can be concentrated efficiently on a core of a small diameter. The configuration of the optical beam diameter reducer is uniform in its longitudinal direction and therefore it can be produced easily and low-priced.